Reptar (CVE-2023-23583) is an Intel CPU bug: a redundant REX prefix in front of rep movsb on processors with Fast Short Repeat Move (FSRM) pushes the core into an unpredictable state.
Tavis Ormandy provides a great technical writeup covering branches to unexpected locations, ignored unconditional branches and machine check exceptions, reproduced from inside an unprivileged guest VM.
Intel have published updated microcode for all affected processors and rate the issue CVSS 8.8 in INTEL-SA-00950.
